Conjoined twins are not only an extremely rare congenital malformation in humans but also one of the most difficult ones to treat. This defect is an example of one of the most complex organisational and ethical issues encountered in medicine.
The incidence of conjoined twins is estimated to be 1 in 50,000 to 1 in 200,000 births. Most conjoined twins are stillborn or die shortly after birth due to accompanying developmental defects. The defect is three times more frequent in girls and its occurrence is higher in Africa and Southwest Asia. There are two contradicting theories to explain the formation of conjoined twins. The most accepted theory is fission; when a single fertilised egg fails to split completely between the 13th and the 15th day after fertilisation. This theory is supported by clinical observations. Conjoined twins are always of the same gender, are genetically identical, and are always joined symmetrically at the same body part. The second theory is secondary fusion of two originally separate embryos.
Conjoined twins are classified according to the site of union, with the suffix ‘pagus’ meaning ‘fixed’. The clinically most useful, although highly simplified, classification of conjoined twins divides them into symmetrically conjoined (the same size, symmetrical to each other) and asymmetrically conjoined (one is always smaller, a parasite), and dependent on the other.
Symmetrically conjoined twins include children joined at the following regions:
Chests (thoracopagus, xiphopagus) with possible fusion involving the heart, liver and upper gastrointestinal tract,
Abdominal cavity (omphalopagus) – united at the region of the liver and gastrointestinal tract where children are facing each other,
Sacral bones (pygopagus) – children with their backs to each other, usually joined by the pelvic organs, anus and pelvic nervous system,
Heads (craniopagus) – usually fused medially sharing the nervous tissues, large vessels and sinuses,
Pelvis (ischiopagus) - most frequently sharing the urogenital system, rectum and liver.
These children can have either 4 fully developed lower limbs or only 3 with 1 fused and that is deformed; this defect may be also accompanied by omphalocoele.
The group of asymmetrical conjoined twins includes cases in which the body parts differ in size - one foetus, called a ‘parasite’, is smaller, partially developed and dependent on the other, fully developed foetus, called an ‘autosite’.